The fundamental mechanisms that regulate the acute inflammatory response in humans with severe sepsis needs to be better defined in order to design specific therapies that can be used to protect the lungs and prevent death from multiple organ failure. Pulmonary macrophages are immune-effector cells that, at least in part, mediate the molecular pathobiology of neutrophilic lung inflammation in response to endotoxin. The overall goal of this proposal is to determine the pathogenic molecular mechanism of the involvement of macrophage gene expression on the development of acute neutrophilic lung inflammation in response to endotoxemia. We hypothesize that the amount and activation status of PU.1 regulates the response of pulmonary macrophage gene expression to endotoxin through enhancement of inflammatory gene production, such as COX-2 and L-PGDS that contributes to the development of cytokine and chemokine-mediated acute neutrophilic lung inflammation. We propose three specific aims: 1) To determine the impact of PU.1 protein levels in macrophages on the development of lung and systemic inflammation, 2) To determine whether the development of neutrophilic lung inflammation is dependent on the activation status of PU.1, and 3) To determine whether the amount and activation status of PU.1 influences the resolution of neutrophilc lung inflammation when resident mature macrophage undergo apoptosis and replacement by infiltrating peripheral blood monocytes that are relatively deficient in PU.1. In the setting of severe sepsis, signaled recruitment and differentiation of macrophages may represent a renewable pool of endotoxin responsive pulmonary macrophages that contribute to the initiation, intensity, and duration of neutrophilic lung inflammation. Our studies are designed to investigate the basic molecular signaling mechanism by which PU.1 is involved in the response of newly recruited and resident pulmonary macrophages to endotoxin with the hope that this leads to novel treatments for ARDS. PUBLIC HEALTH RELEVANCE: The acute respiratory distress syndrome (ARDS) is a severe and life threatening condition that requires a mechanical breathing machine and other supportive care in an intensive care unit. We have shown that macrophages in the lung have an essential role in initiating the severe lung inflammation that causes ARDS. The purpose of this research is to define the basic molecular mechanisms that regulate macrophage involvement in ARDS in order to design new and effective treatments.